1. Field of the Invention
The present invention relates to a device which is arranged to be placed in front of a relatively large surface of an optical element of an instrument, which is located in a dusty environment where settling of the dust readily occurs, said surface of the optical element being exposed to the surrounding atmosphere and said device being intended to keep said surface clean.
2. Brief Description of the Prior Art
Nowadays, an increasingly extensive use is made of optical instruments, such as TV-cameras and optical instruments working within the infrared wavelength region. Particularly, such instrumentation is used more and more for supervising purposes, a TV-camera or an IR-camera being arranged to monitor and register selected changes in the course of events, currently and over a long period of time, which changes in some cases can be controlled by the registration result. However, prior to the present invention, it has been difficult to use instruments of this kind in surroundings where deposits of dust and soot are bound to occur, or in gasfilled and/or damp atmosphere, since the surface of the outermost component of the optical system in the instrument, very often a protecting window, very quickly becomes covered with an opaque layer.
Experiments have earlier been made to solve this problem, by placing a tube in front of the surface of the outermost optical element and by blowing gas in a direction away from said optical element, in order to prevent dust and dirt from impinging up and settling on the surface of the element. The experiments have shown that the tube must be very long in relation to its cross-sectional area, in order to obtain a laminar gas stream therefrom. A laminar stream is essential, since otherwise turbulent flows in the tube will bring dust and dirt into contact with the surface to be kept clean. When there is a demand for a wide view-angle there are, however, contradictory demands on the tube and hence the provision of such a tube is not always a solution to the problem.
Attempts have also been made to solve this serious problem by redrafting the optical properties of the isntruments to be used in such a way, that the front surface of the system is made as small as possible, since it is easier to keep a small surface clean than a larger one. Very often this problem has been solved by placing a focal point of the system at the surface in question. This is not very practical and often impossible and if the optical systems can be constructed without the need to take the dimension of the outer surface into consideration, one restricting parameter can be omitted, which is generally favourable to the system.
The invention overcomes the aforementioned problems. More particularly, the invention inhibits the effect of the very rapidly moving so-called ejector streams which are directed against said surface.